The atomic bomb is the main reason for surrender. Yes other factors contributed but it was atomic bomb that sealed the deal.

The Japanese was extremely stubborn and wanted to keep the war going in hope that a prolonged nasty fight would result in less harsh provisions in the surrender treaty and not resulting in Japanese soldiers being tried as war criminals as they really were. The Bushido Code in World War II mandated that any person subject to Japanese military law should not surrender under all circumstances and that it was better to die for his or her country rather than surrender to enemy soldiers. That's the reason why non-Japanese Asian civilians in occupied nations and American POWs were treated by the Japanese in a rather abusive and inhumane manner systematically. Even after the first atomic bomb, the Japanese didn't believe that we had more than one atomic bomb. They decided to continue the war and hoping to go on as if there wasn't any atomic bomb used against them at all. The second bomb was dropped, and it totally shook the Japanese belief that we had no more. Now, the Japanese realized that we may have two or at least 100 atomic bombs, let alone for the fact they obtained this kind of information from a captured American fighter pilot Marcus McDilda who was shot down two days after we nuked Hiroshima, and was tortured into revealing information how the atomic bomb worked and what cities were the next targets for atomic attacks. In fact, based on the information they obtained from the pilot (who actually told them false information), Japanese war minister Korechika Anami told the weary peace and war cabinet hours after Nagasaki was hit:

"[O]ne atomic bomb could destroy 6 square miles, which was equivalent to 2,000 B-29s each armed with 300 conventional bombs of 500 pounds each . . .The Americans appeared to have one hundred atomic bombs . . . they could drop three per day. The next target might well be Tokyo."

So that was the main reason why the Japanese wanted to surrender, because they didn't like the idea of a weapon that could destroy an entire city in a blink, let alone the fact they feared that with the entire new arsenals of nuclear weapons in our hands, their country and its culture would be destroyed completely. The Japanese decided to surrender unconditionally rather than going on with the fight, in fear of our atomic bombs that could destroy their entire nation and there was nothing they could do to prepare such an attack.

"Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, it would not only result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization. Such being the case, how are we to save the millions of our subjects, nor to atone ourselves before the hallowed spirits of our imperial ancestors? This is the reason why we have ordered the acceptance of the provisions of the joint declaration of the powers." - Emperor Hirohito's surrender speech on radio (the first time he spoke to the Japanese nation with his voice heard all across the Japanese public), August 15, 1945.